Total Strain Amplitude Research Articles

Low cycle fatigue behavior in a medium-carbon carbide-free bainitic steel

In the paper, different morphologies of bainite were obtained through isothermal quenching at 320°C and 395°C in a medium-carbon carbide-free bainitic steel. The cyclic deformation mechanism was explored by using low cycle fatigue testing. The volume fraction of retained austenite was measured by X-ray diffraction and the space partitioning of the solute atoms was constructed by three-dimensional atom probe. Results showed that the fatigue life at 320°C was always higher than that at 395°C under low and high total strain amplitude. The cyclic softening at the early fatigue stage increased the plastic strain of the sample which was responsible for the reduction of the fatigue life at 395°C. Strain-induced retained austenite to martensite contributed to initial cyclic hardening, but almost having no effect on the subsequent cyclic stable/softening behaviors. The finer bainitic ferrite sheaves obtained at 320°C changed the small fatigue crack propagation direction and delayed the crack propagation rate, which was beneficial for the fatigue properties. In addition, the substitutional atoms did not redistribute between the retained austenite and bainitic ferrite before and after cyclic deformation.

Low-Cycle Fatigue Properties of P92 Ferritic-Martensitic Steel at Elevated Temperature

The low-cycle fatigue behavior of P92 ferritic-martensitic steel and the corresponding microstructure evolution at 873 K has been extensively studied. The test results of fatigue lifetime are consistent with the Coffin-Manson relationship over a range of controlled total strain amplitudes from 0.15 to 0.6%. The influence of strain amplitude on the fatigue crack initiation and growth has been observed using optical microscopy and scanning electron microscopy. The formation mechanism of secondary cracks is established according to the observation of fracture after fatigue process and there is an intrinsic relationship between striation spacing, current crack length, and strain amplitude. Transmission electron microscopy has been employed to investigate the microstructure evolution after fatigue process. It indicates the interaction between carbides and dislocations together with the formation of cell structure inhibits the cyclic softening. The low-angle sub-boundary elimination in the martensite is mainly caused by the cyclic stress.

Low Cycle Fatigue Behavior of TA29 Titanium Alloy at Different Temperatures

The low cycle fatigue behaviour of TA29 titanium alloy blisk forging at 300°C, 400°C, 500°C, 600°C were studied. The cyclic stress response, the stress-strain behaviour, the fatigue life and the fractograph were observed and analyzed. Through the double logarithm linear regression, the Coffin-Manson model parameters of TA29 titanium alloy at different temperatures were obtained. The results showed that the fatigue life decreases as the strain amplitude increases. Also, the testing temperature has a significant effect on the low cycle fatigue properties of TA29 titanium alloy. At the same level of the strain amplitude, the fatigue life and the peak value of cyclic stress decrease, while the plastic strain component in total strain amplitude increases with the increase of testing temperature. On the basis of the fractograph of fatigue specimens, the initiation of crack is mainly on the surface of the specimen. Fatigue striations can be seen clearly at fatigue crack propagation area. The fatigue striations in stable fatigue crack propagation area were broadened, and the propagation area was reduced and the final rupture area was increased with the increase of testing temperature.

Strain-controlled cyclic deformation behavior of cast Mg–2.99Nd–0.18Zn–0.38Zr and AZ91D magnesium alloys

This paper investigates the fatigue properties and deformation behavior of the T6-treated Mg–2.99Nd–0.18Zn–0.38Zr (NZ30K04) and an AZ91D-T6 magnesium alloys with the same grain size. Compared with the AZ91-T6 alloy, the NZ30K04-T6 alloy shows significant increases in mechanical properties, achieving 17 MPa in YS, 51 MPa in UTS, 2.4 % in elongation, and 15 MPa in FS, respectively. The NZ30K04-T6 alloy also shows higher cyclic stress amplitudes in comparison with the AZ91-T6 alloy. This is attributed to increased matrix strength from the higher precipitate strengthening in the NZ30K04-T6 alloy. The cyclic stress amplitude of the AZ91-T6 alloy increases with increasing the number of cycles. For the NZ30K04-T6 alloy, the cyclic stress amplitude first increases and then decreases during cyclic deformation. Hysteresis loops were symmetrical at all total strain amplitudes due to the lessened extents of texture in these cast magnesium alloys and the absence of twinning–detwinning during cyclic deformation. For both AZ91-T6 and NZ30K04-T6 alloys, the cracked slip bands initiate the fatigue cracks and also assist the crack propagation. The Coffin–Manson law and Basquin equation can be used to evaluate the fatigue parameters and predict the LCF lives of both AZ91-T6 and NZ30K04-T6 alloys. The multi-scale fatigue (MSF) life models and the same model parameters were also used to predict the HCF lives of these alloys.

The effects of pre-straining conditions on fatigue behavior of a multiphase TRIP steel

To assess the fatigue properties of transformation-induced plasticity (TRIP) steels with various strain histories, a multiphase TRIP780 steel (0.2C–1.7Mn–1.7Si–0.04Al in wt pct) containing 17.3vol pct initial retained austenite was studied. Specimens were pre-strained to 0%, 5%, or 15% in uniaxial tension at room temperature, −20, or 80°C, then subjected to fully-reversed room-temperature fatigue cycling at total strain amplitudes of 0.2–0.6%. The low-cycle fatigue (LCF) life of the TRIP780 steel was relatively independent of prior strain history with respect to the applied strain amplitude. However, the large loss in ductility due to pre-straining to 15% resulted in a decrease in LCF life with respect to the applied plastic strain amplitude. Decreasing the pre-strain temperature (in the range of 80 to −20°C) increased martensite content but did not significantly reduce fatigue life. The amount of austenite formed during fatigue was comparable or slightly larger than the amount formed at equivalent strains in tension.

Low‐Cycle Fatigue Behavior and Life Prediction of Copper Busbar

Copper has many applications especially in rotor parts subjected to fatigue loading. From them, a typical copper busbar is chosen to mainly investigate the low‐cycle fatigue (LCF) behaviors, fracture morphologies, dislocation patterns and crack initiation mechanism. It is found that with total strain amplitude increasing, cyclic softening appears in all the samples; the fatigue crack initiation sites transform mainly from slip bands to grain boundaries; the dislocation spacing of the post‐fatigue samples decreases; by comparison, the Basquin–Manson–Coffin and the simplified hysteresis energy relations are relatively simple and accurate methods to predict fatigue life. Finally, the LCF properties of the coppers with different grain sizes prepared by different technologies are compared and optimized.

Cyclic deformation behaviors of a high strength carbide-free bainitic steel

The cyclic deformation behaviors of low-temperature bainite, lower bainite and upper bainite obtained on a high-strength carbide-free bainitic steel were examined through low-cycle fatigue testing. The relationship between the bainitic microstructure and fatigue behavior was systematically studied using scanning electron microscopy, transmission electron microscopy, atom probe technology, and electron back-scattered diffraction analyses. The results show that low-cycle fatigue undergoes three stages, namely, cyclic hardening, saturation or cyclic softening, and fracturing. The low-temperature bainite exhibits a long fatigue life under the total strain amplitudes, because of its high strength and larger high-angle misorientation distribution of the packets of bainitic ferrite plates. The low-temperature bainite also presents bilinearity in the Coffin–Manson plots. Coordinate deformation and high-uniform elongation lead to prolonged fatigue life at low plastic strain amplitude, whereas phase component primarily affects the fatigue life at high plastic strain amplitude. Highly stable film-like retained austenite is beneficial to arresting fatigue crack propagation. Blocky type retained austenite easily transforms to martensite, resulting in high compatible deformation capability. The hardening ability of the low-temperature bainite is higher than that of the upper bainite, which is attributed to its high pre-existent density of dislocations transformed from movable to immovable during initial hardening stage.

Low cycle fatigue properties of AA2519–Ti6Al4V laminate bonded by explosion welding

Abstract The paper presents the effect of heat treatment on low cycle fatigue properties and the course of fatigue cracking in AA2519/Ti6Al4V sandwich laminates. The laminate is made by explosion welding, using an intermediate layer in the form of an alloy AA1050. The strength properties of bonds were determined during fatigue tests conducted in conditions of constant total strain amplitude eac. This paper describes the impact on the test conditions on the shape of the hysteresis loop, the course of the cyclical strengthening/weakening of the material and the fatigue strength. Test results were extended to the analysis of the surface of the resulting fatigue fractures. The characteristic traits of fatigue cracking were investigated by examining the microstructure of the fracture surface using the scanning electron microscope.

Low Cycle Fatigue behavior of AA7075 with surface gradient structure produced by Ultrasonic Shot Peening

Effect of ultrasonic shot peening (USSP) for different durations, with graded surface structures, was studied on low cycle fatigue behavior of the aluminium alloy 7075 (AA7075) at room temperature. LCF tests were conducted at different total strain amplitudes. Surface microstructures of the USSP treated specimens were characterized using X-ray diffraction and transmission-electron microscopy. No phase change was found due to USSP. Surface region of the ultrasonically shot peened sample was found to develop nanosize grains of 16-20 nm. Significant improvement was observed in LCF life of the specimen subjected to USSP for 180 seconds. However, LCF life of the sample USSPed for long duration of 300 seconds was reduced. The higher fatigue life resulting from USSP for 180 seconds, may be attributed to increased resistance of the USSPed sample against fatigue crack initiation, due to grain refinement in the surface region.

Characterization of Damage During Low Cycle Fatigue of a 304L Austenitic Stainless Steel as a Function of Environment (Air, PWR Environment) and Surface Finish (Polished, Ground)

304L austenitic stainless steel is notably used to manufacture pipes in primary cooling circuits of nuclear power plants operating at a temperature close to 300°C in a specific aqueous environment. Customarily, pipes are roughly polished, however, after some interventions, a few zones might be ground. Start-ups, shutdowns and operating transients of the reactor produce Low Cycle Fatigue (LCF) loadings with different strain rates. These solicitations and the modified surface finish create specific conditions whose effects must be taken into account. In order to understand if there is a joint influence of the PWR environment and surface roughness on the successive stages of fatigue damage leading to failure, fatigue tests were conducted at 300°C under a total strain amplitude of ± 0.6%. Two environments were used, laboratory air and simulated PWR primary aqueous environment. Two surface finishes were investigated, namely fine mechanical polishing and grinding (Rt of about 40μm). The analysis of the damage process is based on data concerning crack density, crack length and striation spacing. The integration of the relation between the striation spacing and the strain intensity factor ΔKɛ, provides an estimation of crack propagation. It is confirmed that in PWR environment one striation can be considered as the crack advance during one fatigue cycle. Moreover, the specific crack front geometry observed in ground specimens and resulting from successive microcrack coalescences explains the reduction in fatigue life induced by grinding.

Fatigue properties of Fe-28Mn-6Si-5Cr-0.5NbC alloy

In this study, we conducted a low-cycle fatigue testing in a Fe-28Mn-6Si-5Cr-0.5NbC (FMS) alloy and a SUS304 steel. The fatigue tests were made using a servo hydraulic fatigue testing machine of capacity 50kN, at maximum total strain amplitudes (εta) of 2.0%, 1.4%, 0.9%, 0.6%. In the SUS304 steel, with decrease in applied strain amplitude, stress-strain hysteresis loop is reduced. On the other hand, the stress response of the FMS alloy is almost unchanged, irrespective of the applied strain amplitude. The life to failure of the FMS alloy is 4 times higher at εta=0.6%, and twice at εta=0.9% and at εta=1.4% than the SUS304 steel. In addition, the stress amplitude of the FMS alloy is 2 times higher at εta =0.6 %, and 1.5 times higher at εta =0.6% than the SUS304 steel. The prolonged fatigue life of the FMS alloy is attributable to the reversible deformation associated with the transformation pseudo-elasticity that can reduce the accumulated strain.

Low cycle fatigue and creep-fatigue interaction behavior of nickel-base superalloy GH4169 at elevated temperature of 650 °C

Total strain-controlled low cycle fatigue (LCF) tests of a nickel based superalloy were performed at 650°C. Various hold times were introduced at the peak tensile strain to investigate the high-temperature creep-fatigue interaction (CFI) effects under the same temperature. A substantial decrease in fatigue life occurred as the total strain amplitude increased. Moreover, tensile strain holding further reduced fatigue life. The saturation phenomenon of holding effect was found when the holding period reached 120s. Cyclic softening occurred during the LCF and CFI process and it was related to the total strain amplitude and the holding period. The relationship between life-time and total strain amplitude was obtained by combining Basquin equation and Coffin-Manson equation. The surface and fracture section of the fatigued specimens were observed via scanning electronic microscope (SEM) to determine the failure mechanism.

Fatigue crack initiation and propagation in lotus-type porous material

The investigation of fatigue strength of lotus-type structure with nodular cast iron as a base material using computational model is analysed in present study. The irregular pores distribution in transversal and longitudinal direction, regarding the external loading, is considered in the computational models. The complete fatigue process of analyzed porous structure is then divided into the crack initiation (Ni) and crack propagation (Np) period where the total fatigue life (N) is defined as: N = Ni + Np. The crack initiation period is determined using strain life approach where elastic-plastic numerical analysis is performed to obtain the total strain amplitude in the critical stress fields around the pores. The simplified universal slope method is then used to determine the number of stress cycles, Ni, required for formation of initial cracks. The number of stress cycles, Np, required for crack propagation from initial to the critical crack length is also numerically determined using finite element (FE) models, in the frame of Abaqus computation FEM code. The maximum tensile stress (MTS) criterion is considered when analyzing the crack path inside the porous structure. The performed computational analyses show that stress concentrations around individual pores are higher when external loading is acting in transversal direction in respect to the pore distribution. Therefore, further computational analyses regarding crack initiation and crack propagation period have been done only for pores distribution in transversal direction.

Low cycle fatigue behavior of casting A319 alloy under two different aging conditions

To determine the influence of peak- and over-aging on low-cycle fatigue (LCF) behavior of casting A319 alloy, the strain controlled low-cycle fatigue tests was studied at room temperature under different total strain amplitudes. At lower total strain amplitudes (0.2–0.3%), the A319 alloy presented initial cyclic hardening and then, steady stage and cyclic softening; while at higher total strain amplitudes (0.35–0.4%), it presented continuous hardening in the peak-aged specimens and constant softening in the over-aged specimens. At a given total strain amplitude, the hysteresis loops in the peak-aged specimens were relatively narrower than those in the over-aged specimens. The fatigue life of the peak-aged samples was higher than that of the over-aged samples at any given total strain amplitude. Fractographic morphology indicated that the size of the fatigue crack propagation zone at peak-aged condition was larger than that at over-aged treatment. The crack characteristic in the propagation region and the fast fracture zone also showed some differences between two aging conditions.

Analysis and Modeling of Thermal Signatures for Fatigue Damage Characterization in Ti–6Al–4V Titanium Alloy

Low cycle fatigue (LCF) damage in titanium alloys has been traditionally assessed based on driving parameters such as stress, strain or hysteresis energy. These mechanical parameters sometimes fail to quantify the true contribution of stored damage energy which is the main cause of LCF damage in these alloys. Hence in the present investigation, thermal evolution captured on-line using lock-in infrared-thermography has been used to characterize low cycle fatigue damage of Ti–6Al–4V under a range of applied total strain amplitudes (0.8–1.8 %). The analysis of thermal signatures indicated that the contribution of thermo-elasticity decreased and inelasticity increased with an increase in strain amplitude due to significant change in damage micromechanism from quasi-cleavage to ductile mode. Increase in applied strain resulted into increased fatigue damage and consequently higher temperature increase. An inverse relationship similar to standard strain amplitude-fatigue life equation was established between temperature increase and fatigue life. The damage energy was evaluated using first law of thermodynamics i.e. law of energy conservation. Damage energy increased with an increase in applied strain amplitude. A relationship was also developed between damage energy and strain amplitude.

Low cycle fatigue behavior of a 10Cr–2W–Mo–3Co–NbV steel

Low cycle fatigue (LCF) behavior of a 10% Cr–2% W–0.7% Mo–3% Co–NbV steel with 0.008wt.% B and 0.003wt.% N additions was studied under fully reversed tension–compression loading as a function of temperature from 20 to 650°С and constant strain amplitude from ±0.2% to ±1.0%. The effect of LCF on the steel’s microstructure was analyzed. It was demonstrated that the 10% Cr steel was susceptible to cyclic softening, even at room temperature, and could not withstand reversed plasticity. The fatigue lifetime curves at all temperatures could be described by the Basquin–Manson–Coffin relationship. The cyclic softening effect was attributed to the coarsening of the laths, primarily at 20 and 500°C, and the transformation of the tempered martensite lath structure (TMLS) to a subgrain structure at 600 and 650°C. In addition, a decrease in the dislocation density occurred under LCF. Increasing the reversed plasticity with the total strain amplitude accelerated the evolution of the TMLS and thus promoted cyclic softening. The propagation of fatigue cracks during Stage II was also facilitated by increasing the contribution of plastic deformation processes to the total strain amplitude, which led to a drop in the fatigue life with an increase in the constant strain amplitude.

Low cycle fatigue behavior of T4-treated Al–Zn–Mg–Cu alloys prepared by squeeze casting and gravity die casting

Gravity die casting (GC) and squeeze casting (SC) T4-treated Al–7.0Zn–2.5Mg–2.1Cu alloys were employed to investigate the microstructures, mechanical properties and low cycle fatigue (LCF) behavior. The results show that mechanical properties of SC specimens are significantly better than those of GC specimens due to less cast defects and smaller secondary dendrite arm spacing (SDAS). Excellent fatigue properties are obtained for the SC alloy compared with the GC alloy. GC and SC alloys both exhibit cyclic stabilization at low total strain amplitudes (less than 0.4%) and cyclic hardening at higher total strain amplitudes. The degree of cyclic hardening of SC samples is greater than that of GC samples. Fatigue cracks of GC samples dominantly initiate from shrinkage porosities and are easy to propagate along them, while the crack initiation sites for SC samples are slip bands, eutectic phases and inclusions at or near the free surface.

High Temperature Low Cycle Fatigue Characteristics of Grit Blasted Polycrystalline Ni-Base Superalloy

The present work is focused on the study of low cycle fatigue behavior of grit blasted nickel-base superalloy Inconel 713LC (IN 713LC). Grit blasting parameters are obtained. Button end specimens of IN 713LC in as-received condition and with grit blasted surface were fatigued under strain control with constant total strain amplitude in symmetrical cycle at 900 °C in air. Hardening/softening curves, cyclic stress-strain curve and fatigue life data of both materials were obtained. Both materials exhibit the same stress-strain response. It has not been observed any improvement or reduction of low cycle fatigue life in representation of total strain amplitude versus number of cycles to failure of grit blasted material in comparison with as-received material. Surface relief and fracture surface were observed in SEM. The little effect of surface treatment on fatigue characteristics is discussed.

Effect of Heat Treatment Process on Microstructure and Fatigue Behavior of a Nickel-Base Superalloy.

The study of fatigue behaviors for nickel-base superalloys is very significant because fatigue damage results in serious consequences. In this paper, two kinds of heat treatment procedures (Pro.I and Pro.II) were taken to investigate the effect of heat treatment on microstructures and fatigue behaviors of a nickel-base superalloy. Fatigue behaviors were studied through total strain controlled mode at 650 °C. Manson-Coffin relationship and three-parameter power function were used to predict fatigue life. A good link between the cyclic/fatigue behavior and microscopic studies was established. The cyclic deformation mechanism and fatigue mechanism were discussed. The results show that the fatigue resistance significantly drops with the increase of total strain amplitudes. Manson-Coffin relationship can well predict the fatigue life for total strain amplitude from 0.5% to 0.8%. The fatigue resistance is related with heat treatment procedures. The fatigue resistance performance of Pro.I is better than that of Pro.II. The cyclic stress response behaviors are closely related to the changes of the strain amplitudes. The peak stress of the alloy gradually increases with the increase of total strain amplitudes. The main fracture mechanism is inhomogeneous deformation and the different interactions between dislocations and γ′ precipitates.

Exponential Model for Damage Accumulation in Closed Cell Aluminum Foams

In this paper the authors propose a new damage accumulation model based on an exponential approach for aluminum closed cell foam under fully reversed cyclic loading. The model developed need as an input, the information about the fatigue behavior of the material and the definition of the failure criterion. In this research, the failure criterion considered is the proposed by Ingraham et al. (2009), and the model to analyze the fatigue behavior is the well-known statistical Weibull model because this model allow us to us directly the total strain amplitude instead of its plastic and elastic components to analyze the fatigue behavior of the material. The model proposed it is a very simple mathematical expression that will allow us to model the damage accumulation level only as a function of the total strain amplitude. Finally, the proposed model has been validated through a comparison of the experimental damage accumulation curves, generated with the previous experimental data published by Ingraham et al. (2009), and the curve generated by the proposed model. The results provide a very good fit between the proposed model and the experimental curves.